Problem: In chemistry class, the girl to boy ratio is $4$ to $7$. If there are $21$ boys, how many girls are there?
Answer: A ratio of $4$ girls to $7$ boys means that there is a group of $4$ girls for every group of $7$ boys. If there are $21$ boys, then there are $3$ groups of $7$ boys each. Because we know the ratio is $4:7$ , there must be $3$ groups of $4$ girls each. There is a total of $12$ girls in chemistry class.